Liquid crystal image display systems have been increasingly used for display of graphic, symbolic, and TV pictorial images. Among the advantages of such display devices are high brightness, large scale display capability, compact size, and high resolution. The basic components of a liquid crystal light valve display system include a liquid crystal layer sandwiched between a transparent front electrode and a reflective back electrode. In the off-state, that is with no voltage applied across the electrodes, the liquid crystal is water clear. Light directed through the transparent electrode transmits through the liquid crystal layer, is reflected from the reflective electrode, and exits the transparent window with the result that a viewer sees a bright region on the liquid crystal display. Conversely, in the on-state, that is with a voltage applied across the electrodes, current flowing through the liquid crystal layer creates a turbulence that causes the liquid crystal to appear milky white. The milky white region scatters light away from the viewer who therefore perceives the region as dark or gray. Because the scattering increases with applied voltage, the shade of gray darkens with increasing voltage. This mode of modulation of the transmissivity of liquid crystal material in response to an applied voltage is called the "dynamic scattering mode".
In practical applications, a spatially modulated electric field pattern containing the image information to be displayed is impressed upon the liquid crystal by one of several methods. In one method, a matrix is individually addressable minute electrodes, each electrode forming one picture element, is formed on the back plate of the liquid crystal light valve. Such an approach is described in U.S. Pat. No. 3,824,003 issued to N. J. Koda et al on July 16, 1974; U.S. Pat. No. 3,862,360 issued to H. G. Dill et al on Jan. 21, 1975 and D. J. McGreivy on July 25, 1978. For the sake of completeness, these three patents are incorporated by reference herein.
In another method, the image containing electric field pattern is generated from an input image directed onto a photo-conductive layer located on the back side of the liquid crystal light valve. Such devices are disclosed in U.S. Pat. No. 3,824,002 issued to T. D. Beard on July 16, 1974 and U.S. Pat. No. 3,976,361 issued to L. M. Fraas et al on Aug. 24, 1976, these patents being incorporated by reference herein.
Two ways to use a liquid crystal light valve in display systems are as a projection system in which the liquid crystal layer of the light valve is projected by a projection lens onto a display screen, and as a direct view display in which the viewer observes the liquid crystal layer directly. In a liquid crystal projection display system described in U.S. Patent application Ser. No. 903,348 filed on May 5, 1978 by Lichty et al to issue as a U.S. Patent to the assignee of the present invention, an aspheric concave reflector is used to concentrate light from a light source onto a liquid crystal light valve where it is reflected through an optical aperture in the reflector to a lens which projects an image onto a viewing screen. Although the system provides a high brightness, high contrast image within a small size package, the aspherical mirror can be both difficult and costly to fabricate because of its unconventional surface shape.
An inherent problem encountered when a reflective dynamic scattering mode liquid crystal device is used as a direct view image display is that the contrast of the image is objectionably reduced by excessive levels of incident light from unwanted sources positioned outside the viewing field. Here, contrast is defined as the brightness ratio of the on-state to the off-state. These unwanted sources can be either the periphery of an extended source such as illuminated walls or sky or a discrete source of extraordinary brightness such as the sun. Heretofore, this problem has been solved by a shielding configuration in which the display is tilted to the viewers line of sight so that all specular reflections from the off-state liquid crystal must originate from a pre-selected area. That area is then blocked with a dark shield. An example of such a configuration is disclosed in the direct view image display device of U.S. Pat. No. 4,090,219 issued to M. N. Ernstoff on May 16, 1978. In an arrangement described in FIG. 1 of that patent, a light trap comprising a planar member having a light absorbing surface extends outwardly from the liquid crystal display surface. The display surface is tilted to the viewer's line of sight so that when the liquid crystal is in the transparent off-state, the viewer sees the dark light trap surface reflected from the back plate of the liquid crystal device with the result that the light valve appears black. When the cell is in the scattering on-state, ambient illumination is scattered from it toward the observer and the activated portion of the surface appears white. Unlike the projector device described previously in which the off-state and on-state are bright and dark, respectively, in this device the contrast is reversed because the off-state and on-state are dark and bright, respectively.
Although this display arrangement maintains a high contrast image even under illumination from highly intense or extended sources, the arrangement has several disadvantages, one of them being that the tilt of the liquid crystal display surface causes objectionable foreshortening of the image displayed on the liquid crystal layer. Another disadvantage is the difficulty in designing a light trap geometry which admits the ambient light needed to illuminate the display while providing an acceptably large angular viewing range and still maintain a small device.